In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in both the manufacturing processes and after the materials are in service in finished products. Specifically, non-destructive evaluation (NDE) methods must assess the structural integrity of composite materials. This assessment detects inclusions, delaminations and porosities. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One solution uses an ultrasonic source to generate ultrasonic surface displacements in a work piece which are then measured and analyzed. Often, the external source of ultrasound is a pulsed generation laser beam directed at the target. Laser light from a separate detection laser is scattered by ultrasonic surface displacements at the work piece. Then collection optics collect the scattered laser energy. The collection optics are coupled to an interferometer or other device, and data about the structural integrity of the composite structure can be obtained through analysis of the scattered laser energy. Laser ultrasound has been shown to be very effective for the inspection of parts during the manufacturing process.
However, the equipment used for laser ultrasound is custom-designed and is presently a limiting factor regarding inspection speed. Previous generation lasers used were either flash-lamp pumped rod architectures, diode-pumped slab configurations, or gas lasers.
Flash-lamp pumped lasers are limited to 100 Hz due to the important quantity of heat generated by the flash lamps that create important distortion in the laser media. Additionally, the life of those flash lamps is typically 100 million shots, requiring replacement every few weeks. Consequently, that type of laser is limited to laboratory applications or to applications where low repetition rates are acceptable. In typical ultrasonic inspection of composites, these lasers are slow and expensive to operate. Diode-pumped slabs are much faster (400 Hz is current limit and 1 KHz may be possible) but they use very expensive custom-manufactured diode arrays to pulse-pump the slabs and create a great amount of heat which can induce thermal distortion. Furthermore, diode array lifetimes have historically been a concern due to both high-cost, reliability and thermal distortion. High-power pulsed-diode pumping of a crystal slab introduces thermal distortions into the slab that ultimately limit the waveform quality of the generation laser beam. Wavefront distortion can limit the useful power of a laser and prevent efficient fiber optic delivery of the beam to the target. Heat removal is a significant design issue for both the diode arrays and the slab. Gas lasers can provide very large energy per pulse and possibly at repetition rates exceeding 400 Hz. However, gas lasers capable of such performances are bulky and heavy; their laser pulse parameters are difficult to adjust, and the emission wavelength cannot be modified outside a certain range. Gas lasers able to operate at repetition rates above 1000 Hz would be significantly heavier and bulkier. Another limitation of the gas lasers is the requirement of maintenance every one to three billion shots to change parts and to clean the optics.
It is important to note that all of the various ultrasound generation laser architectures described here are by their nature large and heavy. Therefore, these architectures are unsuited to use in portable laser ultrasound inspection systems for any sort of remote, in-the-field deployment. In addition, because they are so large and heavy, these architectures require substantial robotic fixturing and complex beam delivery systems even when they are deployed in factory environments, all of which greatly increases the initial overall cost of the laser ultrasound inspection system as well as the maintenance costs to keep the inspection system in operation in a production environment.